The present invention relates to a method for decrypting a first message (xe2x80x9ccxe2x80x9d), which is encrypted from a second message (xe2x80x9cmxe2x80x9d) using a public key in the form of a very large number (xe2x80x9cnxe2x80x9d) containing various prime numbers and of a permutation polynomial. The encrypted message is formed from c=P(m) mod n and the secret key using the product of at least two prime numbers.